LCD & VGA Controllers
11-13
GMS30C7201 Data Sheet
11.6 LCD Timing 2 Register
LCD Timing 2 Register (LcdTiming2) contains seven different bit-fields that are used to
control various functions associated with the timing of the LCD controller.
11.6.1 Pixel Clock Source (PCS)
This bit controls the source of the pixel clock. It can either be the video bus clock, or it
can be the VGA clock. Selecting the video bus clock means that the VGA clock can be
used for the 48MHz clock for FastIR when using only the LCD. Selecting the VGA
clock means that the LCD and the VGA can be operated from the same clock when
sharing data.
11.6.2 Pixel Clock Divider (PCD)
The 5-bit pixel clock divider (PCD) field is used to select the frequency of the
LcdCP
clock signal to the LCD panel. PCD can generate a range of
LcdCP
clock frequencies
from LcdClk/2 to LcdClk/33, where LcdClk is the clock selected by LCS. The frequency
of the pixel clock for a set PCD value can be calculated using the following equation:
Note that in the case of the LCD, the pixel clock is
not
the frequency of some nominal
clock rate that individual pixels are output to the LCD. It is the frequency of the
LcdCP
signal. In normal mono mode (4-bit interface), four pixels are output per
LcdCP
cycle,
so the PixelClock is one quarter the nominal pixel rate. In the case of 8-bit interface
mono, PixelClock is one eighth the nominal pixel rate, since 8 pixels are output per
LcdCP
cycle. In the case of color, PixelClock is 0.375 times the nominal pixel rate,
because 2
2
/
pixels are output per
LcdCP
cycle. If the LCD and VGA are operating
concurrently, and sharing DMA data, then in color mode the pixel clock should normally
be
3
/
the VGA clock. To achieve this, PCD should be programmed to the value 0 and
the skip4 bit set to
“
1
”
. The skip4 bit produces a null clock cycle (no high phase) every
fourth clock cycle.
11.6.3 AC-bias Pin Frequency (ACB)
The 5-bit AC-bias frequency (ACB) field is used to specify the number of line clock
periods to count between each toggle of the AC-bias pin (LcdAC). The value
programmed is the number of lines between transitions, minus 1.
The ACB bit field had no effect on LcdAC in active mode. The pixel clock transitions
continuously in active mode and the AC Bias line is used as an output enable signal.
Note
11.6.4 Invert VSync (IVS)
The Invert VSync (IVS) bit is used to invert the polarity of the
LcdFP
signal. When
IVS=1,
LcdFP
is active LOW. When IVS=0,
LcdFP
is active HIGH.
+
(
)
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